Polyamide-imide coated substrate

ABSTRACT

Steel substrate suitable for forming operations which includes a coating thereon wherein the coating includes a corrosion resistant layer of polyamide-imide having a dry film thickness between 1 and 10 μm.

FILED OF THE INVENTION

The present invention relates to a steel substrate comprising acorrosion protective coating and to a method for making the same. Thepresent invention further relates to the use of the steel substrate informing operations to form a part.

BACKGROUND OF THE INVENTION

Galvanized steel is used in applications where rust resistance isneeded, for instance, in the automotive industry where the zinc layerreduces the onset of rust at exposed edges and surfaces.

Additional coating layers are provided to make the galvanised steelsubstrate more aesthetically pleasing and to further protect the steelsubstrate from corrosion. In this respect galvanised steel substratesare first provided with a phosphate coating (2-5 μm) to enhance paintadhesion, by enhancing paint adhesion such phosphate coatings indirectlyenhance corrosion resistance.

The application of the phosphate coating comprises the steps ofdissolving metal phosphate salts in a solution of phosphoric acid andimmersing the steel substrate in the solution for 4-6 minutes. However,before the phosphate coating can be applied, the automotive manufacturerneeds to clean the galvanised steel substrate, such that it is free fromoil, grease, lubricants and rust. This is not trivial and increasesmanufacturing costs. The use of phosphoric acid also introducesenvironmental and safety issues related to the handling and disposal ofsuch solutions.

Organic coatings are often provided on the phosphate coating byelectrophoretic deposition, which is a process that uses electricalcurrent to deposit paint onto a suitable substrate. Theelectro-(organic)-coating typically has a thickness between 7 and 20 μmand acts as a primer for the application of additional paint layers. Theuse of electrophoretic deposition to apply organic coatings increasesthe cost of manufacturing automotive parts due to the high voltages thatare required to apply such coatings.

It is an object of the present invention to provide a coated steelsubstrate having improved corrosion resistance, adhesion properties andlubrication properties.

It is a further object of the invention to provide a coated steelsubstrate having a reduced coating thickness.

It is another object of the invention to provide a process formanufacturing a coated steel substrate in which the steps of cleaningthe substrate, providing a phosphate coating and an electro-coating areavoided.

DESCRIPTION OF THE INVENTION

The first aspect of the invention relates to a steel substrate suitablefor forming operations comprising a corrosion protective coating whereinthe corrosion protective coating comprises a layer of polyamide-imidehaving a dry film thickness between 1 and 10 μm.

The inventors found that the layer of polyamide-imide (PAI) having athickness between 1 and 10 μm exhibits improved corrosion resistance,humidity barrier properties, friction and adhesion properties relativeto coating systems based on a phosphate layer and an electrophoreticlayer, which have a layer thickness of approximately 2 μm and 20 μmrespectively. The present invention therefore offers automotivemanufacturers a significant advantage both in terms of cost andprocessing and materials handling since a single layer ofpolyamide-imide can be applied in a single pass to replace both thephosphate layer and the electrophoretic layer. The inventors have foundthat the thickness of the layer of polyamide-imide should be at most 10μm since with higher thicknesses the layer may delaminate from the steelsubstrate. Another advantage of the present invention is thatanti-corrosion pigments need not be provided to obtain the improvementsin corrosion resistance.

In a preferred embodiment of the invention the PAI layer has a thermalstability between −60° C. and 400° C. meaning that it will not crack orthermally degrade at reduced or elevated temperatures which may berequired for certain forming operations.

In a preferred embodiment of the invention the thermal degradationtemperature of the layer of PAI is at least 250° C., preferably at least400° C.

In a preferred embodiment of the invention the layer of polyamide-imidehas a dry film thickness between 3 and 5 μm. PAI layers having athickness between 3 and 5 μm exhibit excellent performance in respect ofbarrier, adhesion and corrosion resistance properties that can beprovided at reduced cost relative to other thicker coatings. Preferablythe layer of polyamide-imide has a dry film thickness between 2 and 5 μmsince further cost savings can be made by providing 2 μm thick PAIlayers.

In a preferred embodiment of the invention the layer of polyamide-imidehas a coefficient of friction between 0.13 and 0.2.5, preferably between0.13 and 0.2 at a temperature between −10 and 120° C. Automotivemanufactures specify that coefficient of friction CoF values should bebetween 0.13 and 2.5. The coated steel strips according to the inventioncomply with such standards even when no lubricant is applied.

In the automotive industry lubricant is typically required when forminggalvanised steel strips, and in certain instances, it may also benecessary to provide the galvanised steel strips with texture to improvelubricant retention at the steel strip surface. However, providingtexture can be detrimental to both the formability of the galvanisedsteel strip and the appearance of such formed and painted steel parts.Another consequence of providing texture is that the frictioncoefficient will increase relative to galvanised steel strips withouttexture. Advantageously the layer of polyamide-imide is lubricating,which means an additional step of providing a lubricant and/or textureduring subsequent forming and/or stamping operations is not required.

In a preferred embodiment of the invention the difference in frictioncoefficient between the first tool pass and the last tool pass of thelinear friction test is less than 0.2, preferably less than 0.1 and morepreferably less than 0.05. A stable CoF, preferably between 0.13 and0.25, is of importance to automotive manufacturers. A tool pass may bedefined as the number of times a coated strip is drawn between a flattool and a cylindrical tool of the linear friction test apparatus.

In a preferred embodiment of the invention a zinc or a zinc alloycorrosion protective coating is present on the steel substrate,preferably the zinc alloy comprises Zn as the main constituent, i.e. thealloy comprises more than 50% zinc, and one or more of Mg, Al, Si, Mn,Cu, Fe and Cr. Zinc alloys selected from the group consisting of Zn—Mg,Zn—Mn, Zn—Fe, Zn—Al, Zn—Cu, Zn—Cr, Zn—Mg—Al and Zn—Mg—Al—Si arepreferred and afford additional corrosion protection to the underlyingsteel substrate. Such coatings may be applied by hot-dip galvanising,electro-galvanising, galvannealing or by physical vapour deposition(PVD).

In a preferred embodiment of the invention the steel substrate isselected from the group consisting of carbon steel, low carbon steel,high strength steel, advanced high strength steel, boron steel, nickelchromium steel, electrical steel, tin-plated steel, nickel-plated steeland electro-coated chromium steel. Irrespective of whether the layer ofpolyamide-imide is provided on a coated steel substrate or an uncoatedsteel substrate, the layer of polyamide-imide exhibits improved adhesionto the underlying substrate relative to other commercially basedcorrosion protection systems as evidenced by the obtained CoF values.

Preferably the steel substrate is a strip sheet or blank that issuitable for subsequent forming operations.

The second aspect of the invention relates to a method of manufacturinga steel substrate comprising a corrosion protective coating according tothe first aspect of the invention which comprises the steps of:

-   -   a. providing a steel substrate;    -   b. providing a curable coating on the steel substrate, which        curable coating comprises water, polyamide-amic acid and an        amine;    -   c. curing the curable coating to form the corrosion protective        coating comprising a layer of polyamide-imide having a dry film        thickness between 1 and 10μm.

In a preferred embodiment of the invention the curable coating comprisesup to 20 wt % polyamide-amic acid, up to 7 wt % amine and the remainderbeing water. PAI layers having excellent humidity barrier, corrosionresistance and lubrication properties can be obtained by providing acurable coating comprising 5 to 20 wt %, preferably 5 to 10%polyamide-amic acid and 1 to 7 wt %, preferably 1 to 4 wt % amine. Therole of the amine is to aid the dissolution of the polyamide-amic acidin water by neutralising the amic acid groups and forming thecorresponding water soluble salt. Amines used in accordance with theinvention include ammonia, hydroxyl amines such as2,2-Butyliminodiethanol and tertiary amines such as trimethylamine,N,N-dimethyl ethylamine,N,N-dimethyl propylamine, triethylamine or thelike. Surprisingly, it was found that the use of hydroxyl amines alsoimproved coating performance which may be due to the water solubilityand/or hydrogen bonding properties of hydroxyl amines. Other suitablehydroxyl amines include diethylaminoethanol, diisopropanolamine andaminoethylpropanediol. Tertiary hydroxyl amines are preferred althoughthe presence of primary or secondary hydroxyl amines in the curablecoating also improves the corrosion resistance properties of the layerof polyamide-imide. The inventors found that hydroxyl amines having aboiling point of at least 160° C., preferably above 240° C. furtherimproved the corrosion protective properties of the layer ofpolyamideimide. Tertiary hydroxyl amines such as 2,2-Butyliminodiethanolare particularly preferred.

The rate of curing of the polyamide-amic acid can be enhanced by usingtertiary amines. An amine concentration between 1 and 7 wt % isparticularly suitable for the purposes of aiding dissolution andincreasing the rate of curing. Preferred tertiary amines includetrimethylamine, N,N-dimethylethylamine, N,N-dimethylpropylamine,triethylamine or the like.

In a preferred embodiment of the invention the curable coating is curedusing near infrared radiation which causes the steel substrate to heatup and transfer heat to the curable coating. This has the advantage thatthe curable coating can be cured in seconds rather than minutes.

In a preferred embodiment of the invention the curable coating is curedbetween 160 and 300° C., preferably between 180 and 265° C. Using thispreferred temperature range provides for a more economic curing processto form a cross-linked network of polyamide-imide.

According to a third aspect of the invention the steel substrateaccording to the first aspect of the invention is used in formingoperations to form a part, which include but are not limited to,stamping, blanking and drawing.

In a preferred embodiment of the invention the forming is carried outwithout lubricant. Advantageously the use of the coated steel stripreduces manufacturing costs since additional lubrication is not requiredduring, for instance, deep-drawing or stamping operations. The coatedsteel strip can also be manufactured in-house at the steel stripsupplier. This has a significant advantage for the automotivemanufacturer since the number of manufacturing steps required to formthe part is reduced. At present the automotive manufacturer has to applylubricant on the bare steel strip it has received, form it, clean it,provide a phosphate layer on the steel strip and provide anelectro-coating on the phosphate layer before subsequent paint layerscan be applied. The present invention avoids the automotive manufacturerhaving to apply lubricant before forming, removing lubricant afterforming and having to provide the phosphate layer and electro-coating.

EXAMPLES

The invention will now be elucidated by referring to the non-limitativeexamples below:

Example 1 Preparation of Aqueous Polyamide-Amic Acid Solution

200 g of polyamide amic acid (TORLON® Al-50 from Solvay AdvancedPolymers, available as wet powder with a solids content of 35 wt %, isdispersed in a mixture of water (485 g) and dimethyl ethanol amine (50g). The dispersion is then warmed to a temperature between 60 and 70° C.until the solids dissolve to produce a brown aqueous solution havingabout 9% by weight polyamide-amic acid.

Example 2 Application of the Aqueous Polyamide-Amic Acid Solution

The aqueous polyamide-amic acid solution is cooled to room temperatureand 0.5 wt % of wetting agent BYK® 380N (from Byk Chemie) is addedthereto. This solution is applied on a galvanised steel strip by rollercoating at a speed of 150-250 m/min. The applied coating is then curedvia near-infrared at s temperature of 260° C. for 5-10 s to remove theaqueous solvent and form a layer of polyamide-imide on the galvanisedsteel strip surface.

Experiments

In the experiments hereunder the layer of polyamide-imide has beenapplied on a galvanised steel strip (E1).The present invention (E1) hasbeen compared to a number of commercial coating systems that are used inthe automotive industry to reduce the effects of corrosion caused bysurface defects, scratches and cut edges. C1-C3 are comparative examplesin which galvanised steel strips have been provided with titaniumphosphate (C1), a layer of acrylic containing chromate pigments (Cr 3⁺)thereon (C2) and polyester paint (C3).

Other polyamide amic acid solutions were prepared according to themethod of Example 1 by replacing dimethyl ethanol amine with2,2-Butyliminodiethanol (E2), diisopropanolamine (E3) oraminoethylpropanediol (E4). Dimethyl ethanol amine was also replaced bytrimethylamine as a further comparative example (C4). These solutionswere then applied and cured in accordance with the method of Example 2.

Experiment 1: Salt Spray Test

In order to evaluate the performance of the layer polyamide-imide inrespect of its corrosion resistance the coated steel strip of theinvention was subjected to a salt spray test. The salt spray test wasperformed according to ASTM B117, using a 5% NaCl solution at 35° C.,with an overpressure of 2-3.5 mbar (200 to 350 Pascal) to create foginside the spray chamber.

Table 1 shows the corrosion resistance properties of a galvanised steelstrip provided with a layer of polyamide-imide (E1-E4) and comparativeexamples C1-C4. All thicknesses relate to the thickness of therespective layers, i.e. exclusive of the underlying galvanised coatinglayer. It can be seen from Table 1 that the present invention El offersa significant improvement in corrosion resistance relative to C1-03 withjust 2% white rust being observed after 15 days and 5% white rust beingobserved after 25 days. Similar results are obtained for examples E2-E4.The observed improvement in corrosion resistance properties for E2relative to El has been attributed to the presence of2,2-Butyliminodiethanol having a boiling point of 274° C., which isabove the temperature required to cure the applied coating.

TABLE 1 White rust percentage (Wr %) Thickness (μm) 5 days 10 days 15days 20 days 25 days E1 6 0 0 2 3 5 E2 6 0 0 0 1 3 E3 6 0 0 1 3 6 E4 7 00 4 7 10 C1 4 5 15 26 60 87 C2 4 10 18 40 75 96 C3 14 0 5 18 26 40 C4 87 15 29 32 45

Experiment 2: Cyclic Humidity Test

In order to evaluate the performance of the layer of polyamide-imide inrespect of its humidity barrier properties the coated steel strip of theinvention was subjected to a cyclic humidity test (DIN-norm 50017,IS06270-2) The following conditions were used: 38° C. with a humiditycycle of 100% for 8 hours and atmospheric humidity for 16 hours.

Table 2 shows the humidity barrier properties of the present invention(El) and comparative examples C1-C3. The results show that the layer ofpolyamide-imide is particularly effective as a humidity barrier since nowhite rust is observed after 9 weeks and only 2% white is observed after15 weeks of the cyclic humidity test. For comparative examples C1-C3white rust can be seen after 3 weeks and between 6 and 15 weeks theamount of white rust significantly increases, particularly with respectto C1 and C2.

TABLE 2 White rust percentage (Wr %) Thickness (μm) 3 wks 6 wks 9 wks 12wks 15 wks E1 6 0 0 0 1 2 C1 4 1 4 18 60 87 C2 4 40 80 90 100 100 C3 145 13 20 27 49

Experiment 3: Linear Friction Test

In order to probe the susceptibility of the layer of polyamide-imide togalling the coated steel strip of the invention and the comparativeexamples were subjected to a linear friction test (LFT). Coated steelstrips having a width of 50 mm and a length of 300 mm were oiled whereappropriate (strip E only) with Quaker N6130 oil. The oil had a layerthickness of 1.0±0.2 g/m^(2,)which corresponds to what is usual in apress line. The coated strips of the present invention (strips A-B andcomparative coated strips C-D) were tested dry (no oil) to assess theirself-lubricating properties. The coated steel strips were then pulled ata speed of 0.33 mm/s between a flat tool and a cylindrical tool pushedtogether with a force of 5 kN. The tool material used was DIN 1.3343 andthe surface roughness (Ra) of each tool was 0.4 μm. Before each test thetools were cleaned with a tissue soaked in acetone or alcohol. Thecoated strips were drawn through the tools ten times along a testingdistance of 55 mm; after each stroke the tools were released and thestrips returned to the original starting position in preparation for thenext stroke. All tests were conducted in triplicate at 200 and 80° C. atatmospheric pressure.

FIG. 1 shows the coefficient of friction as a function of the number oftooling steps at 20° C. Strip A is a galvanised steel strip that hasbeen provided with a layer of polyamide-imide having a thickness of 6μm. Strip B is a cold-rolled steel strip that has been provided with alayer of polyamide-imide having a thickness of 7 μm. Strips C-Dcorrespond with comparative examples C1-C2 respectively and strip E is agalvanised strip that has been provided with oil as a lubricant.

From FIG. 1 it can be seen that the difference in coefficient offriction between the first tool pass and the tenth tool pass is lessthan 0.02, i.e. it is substantially constant for both Strip A and StripB of the invention. Moreover, the layer of polyamide-imide is smoothenough to have a friction coefficient that satisfies the CoFrequirements of the automotive industry (0.13-2.5). The results alsoshow that the layer of polyamide-imide has very good adhesion to theunderlying substrate irrespective of whether it is provided ongalvanised steel or cold-rolled steel. If this was not the case then thefriction coefficient would increase with the number of tool passes.During the LFT no tool damage or tool fouling was observed. The layer ofpolyamide-imide is also hard enough to resist contact with a metallictool (at least 10 times). It is this combination of surfacecharacteristics (hard, smooth and adhesive) that allows the coated steelstrips of the present invention to be formed without the need ofadditional lubrication (oil, wax, hard particles).

FIG. 2 shows the coefficient of friction as a function of the number oftooling steps at 20° C. and 80° C. Strip A and B are galvanised steelstrips that have been provided with a layer of polyamide-imide having athickness of 6 μm which were tested for friction respectively at 20° C.and at 80° C. Strips C-D are galvanised strips that have been providedwith oil as a lubricant and were tested respectively at 20° C. and at80° C.

From FIG. 2 it can be seen that the difference in friction coefficientbetween the first tool pass and the tenth tool pass is less than 0.05for both strip A and strip B of the invention, i.e. at 20° C. and 80° C.Thus, even if the strips are heated up due to contact with warm toolingor due to friction during forming, the CoF of the strips with thepolyamide-imide layer still satisfy the requirements of the automotivemanufacturers. In contrast, galvanized steel strips with lubricant,exhibit a CoF increase of up to 350% from pass 1 to pass 10 at 80° C.FIG. 2 also shows that the adhesion of the polyamide-imide coating tothe galvanised steel strip does not degrade with an increase intemperature.

Experiment 4: Adhesion Test

Coating adhesion was assessed in accordance with ASTM D 3359-08, whichcomprises the steps of applying and removing pressure-sensitive tapeover cuts made in the coating. This standard test was carried out on agalvanised steel strip provided with a layer of polyamide-imide (E1) anda galvanised steel strip provided with a phosphate layer and anelectro-(organic)-coating on the phosphate layer (C5). Adhesionproperties were assessed by measuring the amount of coating thatdelaminates from the coated surface after removal of thepressure-sensitive tape. The test results showed that 0% (5B rating) ofthe polyamide-imide layer (E1) delaminated from the galvanised steelstrip surface, whereas 35% (2B rating) delamination was observed for thephosphate/E-coat system (C5).

1. A steel substrate suitable for forming operations comprising acorrosion protective coating wherein the corrosion protective coatingcomprises a layer of polyamide-imide having a dry film thickness between1 and 10 μm, and wherein the layer of polyamide-imide further comprisesa hydroxyl amine.
 2. The steel substrate according to claim 1, whereinthe hydroxyl amine comprises diethylaminoethanol, diisopropanolamine oraminoethylpropanediol.
 3. The steel substrate according to claim 1,wherein the hydroxyl amine has a boiling point of at least 160° C. 4.The steel substrate according to claim 1, wherein the layer ofpolyamide-imide has a dry film thickness between 2 and 5 μm.
 5. Thesteel substrate according to claim 1, wherein the layer ofpolyamide-imide has a coefficient of friction between 0.13 and 0.25, ata temperature between −10 and 120° C.
 6. The steel substrate accordingto claim 1, wherein the thermal degradation temperature of the layer ofpolyamide-imide is at least 250° C.
 7. The steel substrate according toclaim 1, wherein the steel substrate comprises a strip, sheet or blank.8. The steel substrate according to claim 1, wherein the steel comprisesa cold-rolled steel.
 9. The steel substrate according to claim 1,wherein a zinc or a zinc alloy corrosion protective coating is presenton the steel substrate.
 10. The steel substrate according to claim 1,wherein the steel is selected from the group consisting of carbon steel,low carbon steel, high strength steel, advanced high strength steel,boron steel, nickel chromium steel, electrical steel, tin-plated steel,nickel-plated steel and electro-coated chromium steel.
 11. A method ofmanufacturing a steel substrate according to claim 1, which comprisesthe steps of: a. providing a steel substrate; b. providing a curablecoating on the steel substrate, which curable coating comprises water,polyamide-amic acid and a hydroxyl amine; c. curing the curable coatingto form the corrosion protective coating comprising a layer ofpolyamide-imide having a dry film thickness between 1 and 10 μm.
 12. Themethod of manufacturing a steel substrate according to claim 11, whereinthe curable coating comprises up to 20% polyamide-amic acid, up to 7%hydroxyl amine, and water.
 13. The method of manufacturing a steelsubstrate according to claim 11, wherein the curable coating comprises 5to 20% polyamide-amic acid, 1 to 7% hydroxyl amine, and water.
 14. Themethod of manufacturing a steel substrate according to claim 11, whereinthe curable coating is cured using near infrared radiation which causesthe steel substrate to heat up and transfer heat to the curable coating.15. The method of manufacturing a steel substrate according to claim 11,wherein the curable coating is cured between 160 and 300° C.
 16. Amethod of use comprising forming a part from the steel substrateaccording to claim
 1. 17. The method of use of the steel substrateaccording claim 16, in forming operations to form the part wherein theforming is carried out without lubricant.
 18. The steel substrateaccording to claim 1, wherein the hydroxyl amine has a boiling pointabove 240° C.
 19. The steel substrate according to claim 1, wherein thelayer of polyamide-imide has a coefficient of friction between 0.13 and0.2 at a temperature between −10 and 120° C.
 20. The steel substrateaccording to claim 1, wherein the thermal degradation temperature of thelayer of polyamide-imide is at least 400° C.
 21. The steel substrateaccording to claim 1, wherein a zinc or a zinc alloy corrosionprotective coating is present on the steel substrate, wherein the zincalloy comprises more than 50% zinc and one or more of Mg, Al, Si, Mn,Cu, Fe and Cr.
 22. The method of manufacturing a steel substrateaccording to claim 11, wherein the curable coating is cured between 180and 265° C.